1. Field of the Invention
The invention is generally related to bottom-founded offshore platforms and more particularly to such platforms wherein a compliant response to design environmental forces is desirable.
2. General Background
The oil and gas industry has developed a variety of structures, floating vessels, and sub-sea installations to assist with and support drilling and production operations. One of the more common structures used for this purpose is a fixed offshore platform. These platforms have been constructed in an assortment of structural configurations. The term fixed offshore platform in the more general sense is known within the industry to mean any structure founded on the seafloor and extending from the seafloor through the water surface and may support facilities for either drilling and production equipment or both. The portion of the platform housing drilling and production equipment is typically referred to as the platform topsides or deck. The portion of the platform extending from the seafloor through the water surface and supporting the topsides is typically of a type referred to as a jacket (tubular space frame), guyed platform, or tension leg platform. The most common type being the jacket or tubular space frame.
Platforms located in shallow waters are designed for static wind and wave loadings plus dynamic amplification of these loads with little effort directed towards controlling the magnitude of dynamic amplification of these loadings. A platform designed in this manner is generally known within the industry as a `fixed` or `non-compliant` platform. Alternatively, the design of the platform may include measures to limit the degree of dynamic amplification of the applied loads. These structures are generally known within the industry as compliant structures. Platform designs include various techniques and means to introduce compliant behavior. The primary objective of a compliant platform is to provide a structure with natural periods of vibration that are substantially different than the period of the waves containing maximum energy within the design wave spectrum. For offshore platforms located in the Gulf of Mexico, the period of vibration to be avoided is typically 13 to 14 seconds. Most compliant structures are configured to have a period of the first mode of vibration (sway mode) in excess of 25 seconds. It is generally desirable for the second mode of vibration (bending mode) to be less than 8 seconds. These values may vary depending on platform location. Platforms with these vibration characteristics avoid design problems associated with resonance and minimize dynamic amplification of the design loads.
Compliant platforms founded on the seafloor and containing a support structure extending from the seafloor to above the water for the support of topside facilities can generally be grouped into one of the two following groups.
The first of these groups consists of several platform designs wherein a sufficiently long period of the first mode of vibration is provided through some form of articulation of a rigidly framed support structure. This articulation may be located at either the platform base or at some intermediate location between the seafloor and the water surface. Some platforms may also include the addition of a mass trap near the top of the structure to assist with obtaining desired first mode periods of vibration.
One such structure is a guyed platform. Guyed platforms are typically supported vertically and laterally at the base while free to rotate out of vertical about the base. Stability is supplied to the platform by an array of guy lines attached towards the platform top and anchored to the seafloor some distance away from the platform base. The platform is restored to a vertical position after being deflected horizontally by tension forces within the attached guys.
Other compliant platforms of this first group include rigidly framed jackets with a point of minimal rotational restraint at either the seafloor or at some intermediate point typically in the lower half of the support structure. Various spring elements, buoyancy, guys, or a combination of these features provide stability and the capacity to return to a vertical position after being deflected laterally. One platform of this type is disclosed in U.S. Pat. No. 4,696,603. Another similar compliant platform of this type was disclosed in the article entitled "Composite Leg Platforms for Deep U.S. Gulf Waters", Ocean Industry, March 1988. The use of a flex pile was disclosed by these designs as one method of providing the required spring characteristics and restoring forces necessary for stability while providing the flexibility required to obtain compliant first mode vibration characteristics. These flex piles are typically rigidly connected to the platform near the platform mid-height and extend into the seafloor or are connected directly to piling extending from the seafloor. At intermediate locations between the seafloor and the upper end of the flex piles, the flex piles pass through guides providing relative axial movement while restraining the flex piles laterally. These guides are necessary to provide shear force transfer between the platform and the foundation and to also increase the compressive buckling strength of the flex piles. These flex piles may be located within the interior of the platform framing or exterior to the platform framing and are generally always framed to the platform legs by guides and an upper rigid connection. Another similar design incorporating a type of flex pile for a compliant concrete structure is disclosed in U.S. Pat. No. 4,793,739.
Also included within this first group of compliant platform structures is a compliant platform disclosed in U.S. Pat. No. 4,797,034 wherein the articulation point is located between a lower platform portion secured to the seafloor and an upper portion supporting the platform topsides. The lower platform section is secured to the seafloor by piling and is without any compliant features. Tubular members secured to both the upper and lower platform section by rigid connections and guides in a manner similar to the flex piles as previously described provide the required flexibility and stability for a compliant structure. Installation considerations will generally dictate that the platform be fabricated in sections different than those delineated by the point of articulation.
A platform disclosed in U.S. Pat. No. 4,738,567 also makes use of a long flexible piling to achieve a period of first mode vibration suitable for a compliant platform. For this platform the flexible pilings are installed through the diameter of large jacket legs. Each leg may contain several piles, which in turn may contain well conductors and casings. Another compliant offshore platform disclosed in U.S. Pat. No. 5,431,512 provides flexible tubular members located within the jacket legs. For this platform each leg contains a single flex tube member which extends beyond the lower end of an upper jacket section leg and is installed into a pre-installed fixed base section secured to the seafloor with piling using conventional offshore methods. These structures provide a compliant platform that pivots at or near the platform base to obtain required sway mode characteristics. The tubular members located within the platform legs provide platform stability and flexibility.
Compliant platforms of this first group achieve compliant characteristics through articulation about the base or at specific locations where hinge devices have been located. The amount of rotation is controlled by the addition of vertical spring elements normally taking the form of elongated vertical tubulars or flex piles. The use of axial flex tubes spanning across the location of a hinge or pivot point as used by some of the disclosed platforms normally requires a platform to pile rigid connection at each end of the axial flex tubes in addition to the normal foundation pile to platform connections. Flex piles as disclosed in U.S. Pat. No. 4,696,603 and the referenced CCLP platform only require one flex pile to platform connection located at the upper end of each flex pile. However, the combined length of foundation pile and the flex pile requires that the pile section below the seafloor be installed prior to platform installation and then spliced with a pre-installed flex pile section extending to the upper flex pile rigid connection. Alternatively it is required that the combined flex pile and foundation pile be spliced during installation and that the rigid flex pile to platform connection be field installed. If the flex piles are not pre-installed on the jacket structure, the location of anodes for cathodic protection from corrosion will be less than optimal. Platforms, which include flex tubes or flex piles require intermediate slip guides each equipped with wear surfaces. Similar wear surfaces are required at corresponding locations on each of the flex tubes or flex piles. Each of these elements increases the complication of the structure and is generally expensive. Typically flex piles, flex tubes, and slip guides are fabricated from materials with higher than normal strength properties. These materials are expensive and may present unnecessary welding difficulties. Non-traditional erection and installation procedures are required for both the hinge elements and flex piles or flex tubes. The concentration of buoyancy provided by pre-installed flex piles can be problematic during the installation launch operation and tends to inhibit design optimization for platform installation.
A second group of compliant platforms consist of structures that are designed to deflect laterally along the length of the structure as opposed to articulation about a designated point or points. These platforms rely on the global shear stiffness properties of the structure to provide a sway mode period of vibration consistent with the requirements of a compliant offshore platform. Internal forces generated by lateral displacement and buoyancy generally provide stability and restoring forces.
The offshore platform disclosed in U.S. Pat. No. 4,117,690 is an example of a compliant platform of the second type. Traditional framing and jacket legs are replaced with large diameter tubulars through which the foundation piling is installed. The platform legs are connected by horizontal framing members at selected levels. The disclosed platform employs a rigid connection between the platform piling and the bottom of the platform legs at the seafloor. These features distinguish the disclosed platform from the first group of structures that provide a point of articulation in order to obtain the required compliant characteristics of the first mode of vibration. The diagonal bracing normally provided within a platform structure to prevent buckling of the platform legs when subjected to compression loads has been eliminated so as to produce a more flexible structure. Local buckling strength of the legs is increased by pile and well conductor guides attached along the inside of the platform legs that serve as longitudinal stiffeners. The disclosed platform also assumes that the platform will be designed to have sufficient buoyancy such that there will exist at least some tension at the bottom of the platform legs. The disclosed platform also provides various techniques and vibration-influencing means in order to achieve compliant characteristics. One such technique is a provision for adjustable ballast compartments located within both braces and legs whereby both the buoyancy and mass of the platform can be varied. Vibration-influencing means such as the use of added stiffness provided by the introduction of X-bracing at selected levels was also disclosed.
The offshore platform disclosed in U.S. Pat. No. 4,117,690 does not require the added components associated with the compliant platforms disclosed in the first group to achieve compliant periods of vibration but does include features which are non-traditional and difficult to construct and maintain over the life of the structure. The extreme lack of diagonal bracing between the platform legs requires the incorporation of features such as the requirement that there be at least some tension at the bottom of the legs and that the legs be large diameter to achieve required buckling strength. The requirement that there be tension at the bottom of the platform legs imposes weight control restrictions and limitations to future platform modifications normally associated with floating structures. The use of ballast compartments throughout the legs and braces imposes costs and risks related to the construction of piping systems, manifolds, and pumps not normally required for conventional platform construction. These features must be maintained throughout the life of the structure. As disclosed in U.S. Pat. No. 4,117,690 stiffeners may be required to provide sufficient local bucking strength to the large diameter legs. The disclosed platform has assumed that the piling will be installed through guides located internally to the legs that will also serve to stiffen the leg's walls. Pile installation through guides attached to the interior of the leg walls may not be practical for any of the platform embodiments that incorporate variable diameter legs. All of the preferred embodiments of the disclosed platform impose further difficulties regarding pile and well string installation through a reduction in leg diameter below the water line. For these conditions platform piles could be pre-driven allowing the platform to be installed by stabbing over pre-driven piles. However, a stab-over procedure would impose fit-up and alignment costs associated with mating to pre-driven piles. All of these pile installation scenarios would incur the risk and uncertainty of completing the rigid connection of the platform leg to the internally located pile without ready access for contingencies. The inclusion of well strings and risers within the platform legs imposes added operational expenses and introduces the hazard of possible explosive gas accumulations within the platform legs.